Flexural plate wave sensor and array

ABSTRACT

A method for manufacturing a flexural plate wave sensor includes the steps of depositing an etch-stop layer over a substrate, depositing a membrane layer over the etch stop layer, depositing a piezoelectric layer over the membrane layer, forming a first transducer on the piezoelectric layer and forming a second transducer on the piezoelectric layer, spaced from the first transducer. The method further includes the steps of etching a cavity through the substrate, the cavity having substantially parallel interior walls, removing the portion of the etch stop layer between the cavity and the membrane layer to expose a portion of the membrane layer, and depositing an absorptive coating on the exposed portion of the membrane layer.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.09/531,970 filed Mar. 20, 2000 now abandoned.

FIELD OF INVENTION

This invention relates generally to a flexural plate wave sensor andarray, and more particularly to a flexural plate wave sensor havingreduced dimensions which enable the array to have an increased densityof sensors on a single silicon wafer.

BACKGROUND OF INVENTION

Flexural plate wave (FPW) devices are gravimetric sensors capable ofdetecting mass changes as small as 10⁻¹¹ g. Typically, FPW devices arebuilt with a bulk micromachining process which produces a thin filmmembrane of silicon or silicon nitride by etching a cavity through theentire thickness of the silicon wafer with a selective process whichdoes not attack the membrane material. However, due to the crystalstructure of the silicon wafer, the cavity produced by this etchingprocess has interior walls which extend through the silicon wafer at anangle of 126° from the membrane. This results in the cavity having anopening at the bottom surface of the substrate which is at least twiceas large as the area of the membrane. Accordingly, the smallest possibleFPW device built utilizing the prior art bulk micromachining process isapproximately 1 mm×1 mm, since, for this one square millimeter of areaon the surface of the silicon wafer, at least twice as much area isrequired on the bottom of the wafer. Therefore, only small numbers ofFPW sensors can be integrated onto the same silicon chip for exposure tothe same environment. For applications which require several sensorswith different coatings, several packaged sensors must be integratedonto a sensor assembly and exposed to a gas or liquid sample stream.This method is only practical for applications requiring less thanapproximately 20 separate sensors.

BRIEF SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a flexural platewave sensor having reduced areal dimensions which enable an array ofsensors to have an increased density of sensors on a single siliconwafer.

It is a further object of this invention to provide such a flexuralplate wave sensor including a sensor membrane having increasedsensitivity.

It is a further object of this invention to provide such a flexuralplate wave sensor in which the sensor membrane is not sealed on oneside, thereby eliminating atmospheric pressure variations in themembrane tension.

It is yet a further object of the invention to provide such a flexuralplate wave sensor in which the absorptive coating is separate from theelectrical components for sensing elements in fluid environments.

It is a yet further object of this invention to provide a flexural platewave sensor array having increased packing density which enables agreater number of sensors to be fit on a single silicon chip.

It is a further object of this invention to provide a method of making aflexural plate wave sensor in which the sensor membrane is not exposeduntil the end of the manufacturing of the sensor.

This invention results from the realization that a truly effectiveflexural plate wave sensor can be obtained by bulk machining the siliconwafer to form a sensor having a cavity with substantially parallelinterior walls, adding an etch stop layer and a membrane layer to thewafer, adding an absorptive coating on the membrane layer andtransducers on the membrane layer opposite the absorptive coating. Thisconstruction facilitates the formation of an array of sensors havingincreased packing density on the silicon wafer.

This invention features a method for manufacturing a flexural plate wavesensor including the steps of depositing an etch-stop layer over asubstrate, depositing a membrane layer over the etch stop layer,depositing a piezoelectric layer over the membrane layer, forming afirst transducer on the piezoelectric layer and forming a secondtransducer on the piezoelectric layer, spaced from the first transducer.The method further includes the steps of etching a cavity through thesubstrate, the cavity having substantially parallel interior walls,removing the portion of the etch stop layer between the cavity and themembrane layer to expose a portion of the membrane layer, and depositingan absorptive coating on the exposed portion of the membrane layer.

In a preferred embodiment, the method may further include the steps ofetching a hole in the piezoelectric layer and forming a ground contacton the silicon membrane layer.

This invention also features a flexural plate wave sensor including abase substrate, an etch stop layer disposed over the base substrate, amembrane layer disposed over the etch stop layer and a cavity havingsubstantially parallel interior walls disposed in the base substrate andthe etch stop layer, thereby exposing a portion of the membrane layer.The flexural plate wave sensor further includes an absorptive coatingdisposed on the exposed portion of the membrane layer within the cavity,a piezoelectric layer disposed over the membrane layer, a firsttransducer disposed on the piezoelectric layer, and a second transducerdisposed on the piezoelectric layer, spaced from the first transducer.

In a preferred embodiment, the first and second transducers may beinterdigitated transducers. The first and second transducers may beformed from TiPtAu or from aluminum. The piezoelectric layer may beformed from aluminum nitride, lead zirconium titanate or zinc oxide. Theetch stop layer may be formed from silicon dioxide or from silicon andthe base substrate may be formed from silicon.

This invention also features a method for manufacturing a flexural platewave sensor including the steps of depositing a sacrificial materiallayer over a silicon substrate, depositing a membrane layer over thesacrificial material layer with the membrane layer covering thesacrificial material layer and contacting the silicon substrate anddepositing a piezoelectric layer over the membrane layer. The methodfurther includes forming a first transducer on the piezoelectric layer,forming a second transducer on the piezoelectric layer, spaced from thefirst transducer, removing the sacrificial material layer to expose aportion of the membrane layer and depositing an absorptive costing onthe exposed portion of the membrane layer.

This invention also features a flexural plate wave sensor including asubstrate, a membrane layer disposed on the substrate, the membranelayer having legs in contact with the substrate and a body portionspanning between the legs. The substrate, a lower surface of the bodyportion and interior surfaces of the legs define a cavity between thesubstrate and the body portion. The flexural plate wave sensor furtherincludes an absorptive coating disposed on the lower surface of the bodyportion of the membrane layer, a piezoelectric layer disposed over anupper surface of the membrane material, a first transducer disposed onthe piezoelectric layer and a second transducer disposed on thepiezoelectric layer, spaced from the first transducer.

In a preferred embodiment, the substrate may be formed from silicon andthe membrane layer may be formed from silicon. The first and secondtransducers may be interdigitated transducers that may be formed fromTiPtAu or from aluminum. The piezoelectric layer may be formed fromaluminum nitride, lead zirconium titanate or zinc oxide.

This invention also features a method for manufacturing a flexural platewave sensor including the steps of depositing a membrane layer on asubstrate having a concave upper surface, thereby forming a cavitybetween an exposed portion of the membrane layer and the substrate,depositing a piezoelectric layer on the membrane layer, forming a firsttransducer on the piezoelectric layer, forming a second transducer onthe piezoelectric layer, spaced from the first transducer, anddepositing an absorptive coating on the exposed portion of the membranelayer within the cavity.

This invention also features a flexural plate wave sensor including asubstrate having a recess disposed in an upper surface thereof, amembrane layer disposed on the upper surface of the substrate, a cavitydisposed between a portion of the membrane layer and the recess in thesubstrate and a piezoelectric layer disposed on the membrane layer. Theflexural plate wave sensor further includes a first transducer disposedon the piezoelectric layer, a second transducer disposed on thepiezoelectric layer, spaced from the first transducer and an absorptivecoating disposed on the portion of the membrane layer within the cavity.

In a preferred embodiment, the substrate may be formed from a materialselected from silicon or PYREX® material. The first and secondtransducers may be interdigitated transducers formed from TiPtAu or fromaluminum. The piezoelectric layer may be formed from aluminum nitrate,lead zirconium titanate or zinc oxide.

This invention also features a flexural plate wave sensor arrayincluding a substrate, and a plurality of flexural plate wave sensors.Each sensor includes a cavity formed in the substrate, a thin filmmembrane layer spanning the cavity, a piezoelectric layer disposed onthe thin film membrane layer, a transducer disposed on the piezoelectriclayer and an absorptive coating disposed on the thin film membrane layerwithin the cavity. The cavity of each of the sensors includes interiorwalls that are substantially parallel to each other and to the interiorwalls of adjacent sensors.

This invention also features a flexural plate wave sensor arrayincluding a substrate and a plurality of flexural plate wave sensors.Each sensor includes a cavity formed in the substrate, a thin filmmembrane layer spanning the cavity, a piezoelectric layer disposed onthe thin film membrane layer, a transducer disposed on the piezoelectriclayer and an absorptive coating disposed on the thin film membrane layerwithin the cavity. The distance between adjacent sensors is no greaterthan 0.9 mm.

This invention also features a flexural plate wave sensor arrayincluding a substrate, a plurality of flexural plate wave sensors, eachsensor including a cavity formed in the substrate, a thin film membranelayer spanning the cavity, a piezoelectric layer disposed on the thinfilm membrane layer, a transducer disposed on the piezoelectric layerand an absorptive coating disposed on the thin film membrane layerwithin said cavity; a reference flexural plate wave sensor including acavity formed in the substrate, a thin film membrane layer spanning thecavity, a piezoelectric layer disposed on the thin film membrane layerand a transducer disposed on the piezoelectric layer; and amicroprocessor electrically connected to each of the plurality offlexural plate wave sensors and the reference flexural plate wavesensor, for monitoring resonant frequency characteristics of thesensors.

In a preferred embodiment, the reference sensor may monitor the effectsof environmental factors on the sensors and the microprocessor adjuststhe resonant frequency of the sensors to compensate for theenvironmental factors.

This invention also features a flexural plate wave sensor arrayincluding a substrate and a plurality of flexural plate wave sensors,each sensor including a cavity formed in the substrate, a thin filmmembrane layer spanning the cavity, a piezoelectric layer disposed onthe thin film membrane layer, a transducer disposed on the piezoelectriclayer and a plurality of discrete absorptive coatings disposed on thethin film membrane layer within the cavity. The cavity of each of thesensors includes interior walls which are substantially parallel to eachother and to the interior walls of adjacent sensors.

This invention also features a flexural plate wave sensor arrayincluding a substrate; a plurality of flexural plate wave sensors, eachsensor including a cavity formed in the substrate, a thin film membranelayer spanning the cavity, a piezoelectric layer disposed on the thinfilm membrane layer, a transducer disposed on the piezoelectric layerand an absorptive coating disposed on the thin film membrane layerwithin the cavity; a drive amplifier which receives a drive input andoutputs an amplified drive output; a multiplexer which receives theamplified drive output and a selection signal, for driving one of theplurality of flexural plate wave sensors; and an output amplifier whichsenses an output from the one of the plurality of flexural plate sensorsand outputs an amplified sensed signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages will occur to those skilled inthe art from the following description of a preferred embodiment and theaccompanying drawings, in which:

FIG. 1A is a cross-sectional side view of a prior art bulk machinedflexural plate wave sensor;

FIG. 1B is a top view of the prior art bulk machined flexural plate wavesensor;

FIGS. 2A-2H are cross-sectional side views showing the steps involved inthe method of manufacturing the flexural plate wave sensor in accordancewith the present invention;

FIG. 3 is a schematic diagram of a flexural plate wave sensor array inaccordance with the present invention;

FIGS. 4A-4F are cross-sectional side views showing the steps involved inthe method of manufacturing a second embodiment of the flexural platewave sensor of the present invention;

FIG. 5 is a schematic diagram of a flexural plate wave sensor array inaccordance with the second embodiment of the present invention;

FIGS. 6A-6G are cross-sectional side views showing the steps involved inthe method of manufacturing a third embodiment of the flexural platewave sensor according to the present invention; and

FIG. 7 is a schematic diagram of the input and output circuitry of aflexural plate wave sensor array in accordance with the presentinvention.

PREFERRED EMBODIMENT

Flexural plate wave (FPW) sensors are used to sense pressure,acceleration, density of liquids, viscosity of liquids, to detectchemical vapors and to detect biochemical interactions. The surfacemicromachined FPW sensor includes a thin film membrane, a piezoelectriclayer over the membrane, an input interdigitated transducer (IDT)disposed on the piezoelectric layer adjacent a first section of themembrane and an output IDT disposed on the piezoelectric layer adjacenta second section of the membrane. The input IDT responds to an input viapiezoelectric transduction to send an acoustic plate wave across themembrane where it is received by the output IDT where piezoelectrictransduction creates an output. The velocity of the acoustic plate waveis dependent upon the membrane material and the mass per unit area ofthe membrane. The exposed portion of the membrane is coated with anabsorptive coating in order to provide an indication of the detection ofan analyte. Different absorptive coatings may be used in an array todetect different substances. The change of the mass per unit area causedby the absorption of an analyte by the absorptive coating on themembrane provides a shift in velocity of the acoustic plate wave sentacross the membrane from the input IDT to the output IDT and aconcomitant frequency shift. The shift in frequency is detected,indicating that the target chemical vapor or substance has been detectedby the sensor.

A prior art bulk micromachined FPW device 10 is shown in FIGS. 1A and1B. The FPW device 10 includes a substrate 12 of undoped silicon. Alayer of membrane material 14 is deposited on a surface of the siliconsubstrate 12. A piezoelectric layer 16 is deposited on the layer ofmembrane material 14. An input IDT 18 is disposed on the piezoelectriclayer of material 16 proximate a first portion of the membrane layer 14and an output IDT 20 is disposed on the piezoelectric layer 16 proximatea second portion of the membrane layer 14.

Utilizing bulk micromachining techniques, a cavity 22 is etched into thesilicon substrate 12 such that a section of the membrane layer 14 isexposed. The exposed section of the membrane layer 14 is coated with anabsorptive coating 24 such that absorption by the absorptive coating 24the target substance is detected by the device 10. However, physicallimitations of the bulk micromachining process in the formation ofcavity 22 results in the cavity having interior walls 26 and 28 whichare formed at an angle α of 126°. Accordingly, due to the aspect ratioof the height relative to the width of the cavity 22 etched into thesilicon substrate, the size of the membrane layer 14 can be no smallerthan approximately 1 mm×1 mm, and the minimum spacing between adjacentsensors in an array can be no less than approximately 1 mm.

Given the size limitation of the exposed surface of the membrane layer,in order to detect a large number of components of a gas or liquidstream, a large number of sensors must be provided. The physical size ofa sensor assembly incorporating a large number of sensors must also belarge in order to provide detailed analyses of the fluid being tested.Accordingly, detailed analysis of a sample becomes cumbersome anddifficult to manage since there are multiple large sensors, each ofwhich must be exposed to the same liquid or gas sample.

FIGS. 2A-2G illustrate the steps involved in the method of manufacturingthe flexural plate wave sensor in accordance with the present invention.Shown in FIG. 2A is a silicon-on-insulator (SOI) wafer 30, whichincludes a silicon substrate 32, a silicon dioxide etch stop layer 34 onthe silicon substrate 32 and a silicon membrane layer 36 on the silicondioxide etch stop layer 34. In the preferred embodiment, the siliconsubstrate 32 is approximately 400 microns thick, the silicon dioxideetch stop layer 34 is 1 micron thick and the silicon membrane layer 36is 2 microns thick. It is a portion of this silicon layer 36 which, inthe finished sensor shown in FIG. 2G, is the thin film membrane throughwhich the acoustic plate wave is transmitted.

A piezoelectric layer 38, FIG. 2B, is then applied to the upper surfaceof the silicon membrane layer 36. Piezoelectric layer 38 has a thicknessof 0.5 microns and can be formed from any piezoelectric material, suchas aluminum nitride or zinc oxide. A hole 40, FIG. 2C, is then etchedinto the piezoelectric layer 38 to the surface of the silicon membranelayer 36. A ground terminal 42, FIG. 2D, is deposited in the hole 40, incontact with the silicon layer 36, and input IDT 44 and output IDT 48are deposited on piezoelectric layer 38. Ground terminal 42, input IDT44 and output IDT 48 are preferably formed from a 0.1 micron thick layerof TiPtAu metal. Alternatively, ground terminal 42, IDT 44 and IDT 48may be formed from aluminum. Using an inductively coupled plasma (ICP)etch machine, a cavity 50, FIG. 2E, is etched into the silicon substrate32 up to, but not including the silicon dioxide etch stop layer 34,which acts as an etch stop for the ICP process. The exposed portion 51of the silicon dioxide etch stop layer 34 is then removed by dipping theportion 51 into buffered hydrofluoric acid, thereby exposing a portionof the silicon layer 36 to form thin film membrane 53. As shown in FIGS.2F and 2G, the resulting sensor 60 includes a cavity 52 having interiorwalls 54 a and 54 b which are much less than 126°: they aresubstantially parallel to each other. An absorptive coating 56, FIG. 2G,is then applied to the exposed surface of thin film membrane 53 ofsilicon layer 36.

In operation, the input IDT 44 transmits an acoustic plate wave acrossthe thin film silicon membrane 53 in the direction of arrow 62, where itis received by the output IDT 48. As long as the absorptive coating 56does not absorb any of the target substance, the mass per unit area ofthe membrane 36 remains constant, resulting in a constant frequency ofthe acoustic plate wave. However, as the absorptive coating 56 absorbsthe target substance, the mass per unit area of the membrane increases.This results in a shift in the velocity of the acoustic plate wave and,consequently, a frequency shift in the wave received by the output IDT48. This frequency shift is recognized as an indication that the targetsubstance has been detected by the sensor 60. Alternatively, more thanone type of absorptive coating 56, 56′ may be applied to the membrane 53of each sensor. See FIGS. 2G and 2H. This enables each sensor to detectdifferent analytes which may be absorbed by the different coatings.

An array 70 of flexural plate wave sensors 60 is shown in FIG. 3. Due tothe process described above with reference to FIGS. 2A-2G, the resultingsensor 60 can be made as small as approximately 500 microns by 100microns. Furthermore, since the interior walls 54 a and 54 b of cavity52 are substantially parallel to each other and to the interior walls ofadjacent sensors, these sensors can be more densely packed onto thesubstrate 32. As shown in FIG. 3, the spacing between adjacent sensors60 can be as little as 100 microns. This configuration enables anincreased number of sensors 60 to be fit onto a single silicon wafer,thereby enabling an increased number of substances to be detected withthe use of a single silicon chip.

The steps involved in the method of making a second embodiment of theflexural plate wave sensor are illustrated in FIGS. 4A-4F. The processbegins with a silicon substrate 80, FIG. 4A on which a sacrificial layerof material 82 is deposited, FIG. 4B. The sacrificial layer 82 is anymaterial that can be easily removed from the substrate 80, such as glassor a photoresistive material. A structural layer 84 is deposited overthe sacrificial layer 82, FIG. 4C. The structural layer 84 covers thetop and sides of the sacrificial layer 82 and contacts the substrate 80.Piezoelectric layer 86 is deposited over the structural layer 84, FIG.4D. An input IDT 88 and an output IDT 90 are then deposited on thepiezoelectric layer 86, FIG. 4E. The sacrificial layer 82 is then etchedaway to create a cavity 92 with a thin film membrane 93 disposed betweenthe cavity 92 and the piezoelectric layer 86. An absorptive coating 94is then deposited on the exposed surface of the thin film membrane 93,FIG. 4F. The resulting sensor 100, FIG. 4F is sized similarly to thesensor 60, FIG. 2G, and operates in the same manner.

An array 102 of sensors 100 is shown in FIG. 5, where it can be seenthat, due to this particular method of manufacturing, with the shallowerangled sides on the cavity, the sensors 100 require as little as 100microns between adjacent sensors, thereby increasing the packing densityof the array 102.

A reference sensor 200, which is formed identically to sensors 100, butdoes not include the absorptive coating, is used to monitor the effectsof environmental factors, such as temperature and pressure on theresonant frequency characteristics of the sensors 100. Each of thesensors 100 and sensor 200 is connected to a microprocessor 104 vialines 106 a-106 d. Microprocessor 104 monitors the resonant frequencycharacteristic of sensors 100 and 200 independently, so thatenvironmental factors as sensed by sensor 200 can be compensated for.

The sensor input and output circuitry for the sensor array 102 andgenerally shown at 300 in FIG. 7. Circuitry 300 includes a driveamplifier 302 which is a high-again single-ended input-to-differentialoutput amplifier, which receives an input on line 310 and outputs adifferential signal on lines 312. Multiplexer 304 receives thedifferential outputs on lines 312 and, based on a selection signalpresent on lines 314, selects one of the n sensors 100 of array 102 toactivate by providing a differential drive signal to the selected sensor100 on lines 316. The differential output of the selected sensor 100 isinput to amplifier section 308 on lines 318. Amplifier section 308includes high-again amplifiers 320 that are configured as aninstrumentation amplifier. This configuration allows for symmetricalloading on each sensor output, high common-mode signal rejection, andhigher gains for a given limited bandwidth. Amplifier section 308outputs the amplified sensor output on line 322. This configurationenables a single input/output device to drive and monitor an array ofsensors. The multiplexer 304 can be operated to cycle through eachsensor in the array. Accordingly, an array having a number n of sensors,each having a different absorbtive coating for detecting differentanalytes, can be driven with a single input/output device which cyclesthrough the sensors for detecting the presence of a number of analytes.

FIGS. 6A-6G illustrate the steps involved in the method of making athird embodiment of the flexural plate wave sensor. First, asilicon-on-insulator layer 108, including a silicon layer 110, a silicondioxide layer 112 and a silicon handle wafer 114, FIG. 6A is depositedon a substrate 116. The substrate 116 may be formed from PYREX® brandglass from Corning or silicon, and has a cavity 118 disposed on an uppersurface thereof, FIG. 6B. The resulting structure 117 is shown in FIG.6C. The silicon handle wafer 114 and silicon dioxide layer 112 are thenetched away, leaving the silicon layer exposed, thereby forming a thinfilm membrane 119 over the cavity 118, FIG. 6D. A piezoelectric layer120 is then deposited over the silicon layer 110, FIG. 6E. An input IDT121 and an output IDT 122 are then formed on the piezoelectric layer 120over the silicon membrane 119, FIG. 6F. Finally, an absorptive coating124 is deposited over the bottom surface of the silicon membrane 119 toform the flexural plate wave sensor 130, FIG. 6F. The sensor 130 issized similarly to the sensor 60, FIG. 2G, and operates in the samemanner.

It can therefore be seen, that, due to the manufacturing processesinvolved in fabricating the flexural plate wave sensors of the presentinvention, the size of the membrane can be greatly reduced compared toprior art sensors. This reduction in the size of the thin film membraneallows the thickness of the thin film membrane to be greatly reduced.Since the sensitivity of the sensor is inversely proportional to themass per unit area of the thin film membrane, a membrane which is tentimes thinner than a prior art sensor is consequently ten times moresensitive than the prior art sensor. Furthermore, as described above,the reduction in the area of the sensors combined with the feature ofthe substantially parallel interior walls of the cavities of the sensorsallows a greater packing density of the sensors, resulting in a greaternumber of sensors being disposed on a single silicon chip.

The FPW sensors produced by the above described methods have numerousapplications, including a gas analyzer device capable of detecting thepresence and concentration of hundreds of molecular components with lessthan one part per billion minimum detectable concentration sensitivity.The FPW sensor could be incorporated into a liquid analyzing devicecapable of analyzing samples for several hundred possible contaminantsor components simultaneously. The FPW sensor could also be utilized aspart of a DNA sequencing device, as a virus/antibody detection deviceand for biological weapon detection.

Although specific features of the invention are shown in some drawingsand not in others, this is for convenience only as each feature may becombined with any or all of the other features in accordance with theinvention.

Other embodiments will occur to those skilled in the art and are withinthe following claims:

1. A method for manufacturing a flexural plate wave sensor comprisingthe steps of: depositing an etch-stop layer over a substrate; depositinga membrane layer over said etch stop layer; depositing a piezoelectriclayer over said membrane layer; forming a first transducer on saidpiezoelectric layer; forming a second transducer on said piezoelectriclayer, spaced from said first transducer; etching a cavity through thesubstrate, the cavity having substantially parallel interior walls;removing the portion of the etch stop layer between the cavity and themembrane layer to expose a portion of the membrane layer; and depositingan absorptive coating on the exposed portion of the membrane layer. 2.The method of claim 1 further comprising the steps of etching a hole inthe piezoelectric and forming a ground contact on the silicon membranelayer.
 3. A flexural plate wave sensor comprising: a base substrate; anetch stop layer disposed over said base substrate; a membrane layerdisposed over said etch stop layer; a cavity disposed in said basesubstrate and said etch stop layer, thereby exposing a portion of saidmembrane layer, said cavity having substantially parallel interiorwalls; an absorptive coating disposed on the exposed portion of saidmembrane layer within said cavity; a piezoelectric layer disposed oversaid membrane layer; a first transducer disposed on said piezoelectriclayer; and a second transducer disposed on said piezoelectric layer,spaced from said first transducer.
 4. The flexural plate wave sensor ofclaim 3 wherein said first and second transducers are interdigitatedtransducers.
 5. The flexural plate wave sensor of claim 3 wherein saidfirst and second transducers are formed from TiPtAu.
 6. The flexuralplate wave sensor of claim 3 wherein said first and second transducersare formed from aluminum.
 7. The flexural plate wave sensor of claim 3wherein said piezoelectric layer is formed from a material selected fromthe group consisting of aluminum nitride, zinc oxide and lead zirconiumtitanate.
 8. The flexural plate wave sensor of claim 3 wherein said etchstop layer is formed from silicon dioxide.
 9. The flexural plate wavesensor of claim 3 wherein said membrane layer is formed from silicon.10. The flexural plate wave sensor of claim 3 wherein said basesubstrate is formed from silicon.
 11. A flexural plate wave sensor arraycomprising: a substrate; and a plurality of flexural plate wave sensors,each sensor including a cavity formed in the substrate, a thin filmmembrane layer spanning the cavity, a piezoelectric layer disposed onthe thin film membrane layer, a transducer disposed on the piezoelectriclayer and a plurality of discrete absorptive coatings disposed on saidthin film membrane layer within said cavity; wherein said cavity of eachof said sensors includes interior walls which are substantially parallelto each other and to the interior walls of adjacent sensors.